Painting has long been the process of choice for applying coatings to surfaces, especially those having complex curvature. Painting is generally a controllable, reliable, easy, and versatile process. The paint can include additives to give the surface desired physical properties, such as gloss, color, reflectivity, or combinations thereof. The painting process is well understood and produces quality coatings having uniform properties even when the surface includes complex curvature. However, painting is falling under closer environmental scrutiny because paints use volatile solvents to carry the binder and pigments or because of the binder precursors and pigments themselves. Therefore, there is a need to replace the painting process with a process that has less environmental impact. Furthermore, while painting is well defined, well understood, and common, it remains an “art” where masters produce better products than novices or apprentices without necessarily being able to account for why or to teach others how.
Painted surfaces sometimes lack the durability that quality-conscious customers demand. The surface must be treated and cleaned prior to applying the paint. The environment surrounding the part must be controlled during application of the coating, often requiring a spray booth. Painted coatings are also vulnerable to damage like cracks or scratches. Isolated damage may require the repair of a large area, such as forcing the repainting of an entire panel.
Spraying inherently wastes paint and is unpredictable because of the “art” involved with the application. Improper application cannot be detected until the spraying is complete, then rework to correct a defect usually affects a large area even for a small deficiency. Furthermore, in the aerospace industry, painting requires specialized equipment and facilities that are expensive to construct and to operate. Painting takes an aircraft out of revenue-generating service. Painting can only be done where a paint hangar is available, and is relatively slow and inflexible.
In the context of aircraft, U.S. Pat. No. 4,986,496, the contents of which are incorporated by reference, describes a drag reduction article in the form of a conformable sheet material (a decal) with surface texturing for application to aircraft flow control surfaces to reduce aircraft drag. The material fits on curved surfaces without cracks, bubbles, or wrinkles because of paint-like properties of the basic carrier film. The decals are manufactured flat and are elongated to fit the intended curvature. If the appliqué deformation is not plastic, this elongation can be problematic over time if the stretched material shrinks to expose a gap between adjacent decals where weather can attack the decal-surface interface. Appliqués or decals must be plastically deformable or they will be limited to surfaces of slowly changing curvature.
Appliqués (i.e. decals) are also described in U.S. Pat. No. 5,660,667, the contents of which are incorporated by reference. Having complex curvature, the appliqués form complete, bubble-free, wrinkleless coverings on surfaces of complex curvature without excessive elongation. Lapping of appliqués is generally described in European Patent Application publication no. 1093409, the contents of which are incorporated by reference.
Often surfaces must be protected against corrosion. Such protection commonly involves surface treatments or primers (i.e. chromated primers or conversion coatings) that are relatively expensive because of the chemicals involved and the time associated with their application. These traditional coatings are relatively heavy, especially when coupled with other surface coatings that must be applied over the corrosion protection coating to provide color, gloss, enhanced surface durability, abrasion protection, a combination of these attributes, or other attributes. The chemicals used in conventional corrosion protection coatings often are hazardous materials.
Appliqués are of considerable interest today for commercial and military aerospace applications. Flight tests have been conducted on paintless aircraft technologies, such as appliqués. These appliqués save production costs, support requirements, and aircraft weight while providing significant environmental advantages. Some of these appliqués are described in greater detail in U.S. Pat. No. 6,177,189 and in an article entitled “Paintless aircraft technology,” Aero. Eng'g, November 1997, p. 17, which are incorporated by reference. Further, some commercial airlines, like Western Pacific, use appliqués to convert their transports into flying billboards.
In addition to the above advantages, appliqués incorporating metal layers may also provide protection against lightning strike. A description of an appliqué providing protection against lightning strike is described in U.S. Pat. No. 4,352,142, which is incorporated by reference. Lightning strikes may potentially cause damage to aircraft—especially composite aircraft. A typical lightning strike on an aircraft may initially attach at a location such as a leading edge of an engine inlet cowl or the nose of the fuselage, collectively referred to as Zone 1. An initial Zone 1 lightning strike may be a rapid spike of electrical current with a peak amplitude on the order of around 200 KA that may last for around 500 μSec or so (referred to as an “A” waveform).
As the aircraft flies through the plasma field of the lightning, the lightning may reattach aft of the Zone 1 strike at locations such as an engine exhaust outlet (referred to as Zone 2) or on a wing skin (referred to as Zone 2 or 3 depending on the location). Zone 2 reattachment can experience a continuing current charge transfer of up to about 10 coulombs over a period of time on the order of around 5 milliSec or so (referred to as a “B” waveform). Zone 3 reattachment can experience a continuing current charge transfer of up to about 200 coulombs over a period of time between around 0.25 Sec and around 1 Sec or so (referred to as a “C” waveform).
A restrike can occur at any Zone and is referred to as a “D” waveform. A “D” waveform restrike may be a rapid spike of electrical current with a peak amplitude on the order of around 100 KA that may last for around 500 μSec or so.
A major concern is to protect against a “D” waveform restrike in Zones 2 or 3—especially in the vicinity of a fastener that extends into a wing box that may be wetted with fuel. Another concern is to mitigate damage to composites that may be caused by the continuing currents of the “B” and “C” waveforms.
For example, U.S. Patent Application Publication No. 2002/0081921 by Vargo et al. (the contents of which are incorporated by reference) describes an appliqué that includes a polymeric sheet material, such as a halopolymer fabric, that is adhered to or bonded to a metal layer, such as a metal mesh or an expanded metal foil. The metal layer is adhered directly with an adhesive to a nonmetallic substrate, such as a composite material used in an aircraft structure. In the event of a lightning strike, energy from the lightning is dispersed over a large surface area, thereby mitigating localized damage to the nonmetallic substrate. However, because the metal layer is adhered directly to the nonmetallic substrate, the energy from the lightning strike is maintained in contact with the nonmetallic substrate. As a result, a large surface of the nonmetallic substrate may be placed in contact with large amounts of energy from the lightning strike.
A concern with use of conductive elements such as expanded aluminum or copper foils or interwoven wire fabric (IWWF) incorporated as part of the skin of composite aircraft for lightning protection is cracking due to the differential coefficient of thermal expansion of the expanded metal and the resin/composite. Even micro-cracking in a composite may lead to further cracking and it is disallowed by FAA certification rules.
In addition, damage to internal structure of a composite aircraft and to the interior of the composite structure itself may be difficult to assess and repair, and may present a long-term aging issue for the aircraft. If composite structures are exposed to the high currents typically imparted by a lightning strike, then damage (such as charring, bond-breaking, loss of distortional capability) may occur to the resin. Another concern is reliably predicting where currents will go once an aircraft is struck by lighting. As a result, many areas of the structure are currently over-designed and many protection schemes are duplicated.
Further, lightning protection systems currently in use require a connected electrical path in order to transfer current. However, each fastener and panel joint is a discontinuity that presents an opportunity for the current to bury into and possibly damage the structure below or even ignite fuel carried within. Therefore, it would be desirable to allow the transfer of energy around or over these discontinuities, especially in the case where there is panel-to-panel motion.
It would also be desirable to mitigate effects of static charge developed during flight. As an aircraft flies through the air, electrons in air molecules may be forcibly dislodged from their orbits by impact with the skin of the aircraft. The electrons may be stored on the composite skin of the aircraft and impart a static charge, referred to as a P-static charge. This P-static charge may possibly result in personnel injury if a person were to contact an aircraft skin after landing but before the aircraft were electrically grounded. Further, discharge of the P-static charge may result in electrical noise that can interfere with electronic systems of the aircraft.
Accordingly, it may be desirable to increase lightning strike protection afforded by an appliqué and/or simultaneously mitigate static charging. However, there is an unmet need in the art for a low-cost appliqué that provides increased protection from lightning strike to an underlying surface, and/or that mitigates static charging.